Call broadcast surveys monitor owls with more precision than passive surveys by citizen scientists or acoustic recording units

Context: Population monitoring that effectively detects population changes is an important component of threatened species management. Call broadcast surveys are regularly used for robust monitoring of vocal or territorial species; however, they can be resource intensive. Passive monitoring approaches involving citizen scientists and acoustic recording units may be less costly, and consequently these methods are increasingly being advocated. 

Aims: This study investigated the utility of three different monitoring approaches to detect population trends in the Critically Endangered Norfolk Island morepork Ninox novaeseelandiae undulata. It compared the effectiveness of call broadcast with that of passive citizen scientist and acoustic recording surveys. 

Methods: Between November and December 2020, we trialled each technique on Norfolk Island. The coefficient of variation in the number of moreporks heard per survey, and the mean detectability at each site were calculated as the core metrics for each technique. The statistical power of each technique to detect population trends over time was also calculated to compare efficacy. An extensive island-wide call broadcast survey, roost searching, and GPS tracking program were used to establish an independent population estimate. 

Key results: We established a population estimate of 25 individuals. The call broadcast approach had the highest precision and highest site detectability. This approach was also assessed as being capable of detecting a change of just two or three moreporks per year, after three years of monitoring. By contrast, approaches using passive citizen science surveys and acoustic recording units would require at least ten years of monitoring to detect similar population trends. 

Conclusions: Call broadcast surveys had a greater capacity to achieve high detectability and precise monitoring, relative to the passive approaches. 

Implications: The high precision of call broadcast appears to justify the higher resource requirements relative to other methods examined in this study, where the target species is territorial and extremely rare. Call broadcast surveys are recommended for long-term monitoring of Norfolk Island moreporks.